途中に / 途中で|とちゅうに / とちゅうで

1. 1Partway through an activity or event that has started but not yet finished.Ex. 1
2. 2On the way somewhere, at a point during a journey before reaching the destination.Ex. 2

A neutral expression used in both speech and writing to mark a point within an activity or journey that is still in progress. The activity or trip usually has a clear start and end, such as a meeting, a meal, or going to school. With motion verbs like 行く and 帰る, it often sets the scene for something that happened en route. Unlike 最中に, it is the usual choice for 'on the way' somewhere.

• 最中にN3「最中に」focuses on an action in progress and cannot be used as 「行く最中」 to mean 'on the way'; for movement you must use the ている form, as in 「向かっている最中に」. When an accident happens mid-activity the two are often interchangeable, but 「最中に」 more strongly emphasizes being interrupted.
• ところに / ところへN2「ところに / ところへ」emphasizes that something happens at just that moment, and the following clause is typically an external event such as a visit or phone call, often with a nuance of surprise or disturbance. It does not indicate a point along the way or giving up midway, and it can attach to the た form and adjectives.
• 間にN4「間に」objectively delimits a period of time or state, and can express intentionally completing something before the period ends; it can also attach to the ない form and adjectives. 「途中に / 途中で」focuses on the middle point of an unfinished process or journey, and can also refer to a point along the way.
• たとたんN3「たとたん」means that as soon as the first action occurs, the second follows almost immediately, and it attaches only to the verb た form. 「途中に / 途中で」refers to the middle of an activity or movement that has not yet ended, attaching to a noun + の, the verb dictionary form, or the ている form, and not the た form. The two cannot be interchanged.
• うちにN3「うちに」expresses doing something while the chance lasts ('take advantage of...') or a gradual unnoticed change during a continuing state, and it can attach to adjectives and the ない form. 「途中に / 途中で」refers to the middle point of a process or journey, with the following clause typically being a sudden event or interruption; it cannot express the 'take advantage of the chance' meaning of 「うちに」.

最中に also means 'in the middle of' an activity but stresses that it is in full progress and is not the usual choice for being on the way somewhere, while 途中に / 途中で locates a point between start and end, including en route. ところに / ところへ marks the moment something comes along, 間に marks a span within which something happens, うちに means while a state lasts or before it changes, and たとたん means immediately after an action, so none of these four specifically points to a stage partway through.