The state’s convention center authority has completed the purchase of 5.6 acres of land across from the Boston Convention & Exhibition Center for the development of two mid-priced hotels and parking, the authority said Monday.

The $33 million purchase of the rectangular lot along D Street is the first step toward a broader $2 billion expansion that would include a doubling of the facility’s exhibition space and the addition of at least 1,500 hotel rooms.

Selling the vacant land was Intercontinental Real Estate Corp., which had been planning to build apartments on the property.

The authority is preparing to seek proposals from developers for construction of the hotels, which together would have as many as 500 rooms. The work could start by the end of next year, with an opening planned for 2015.

Officials said the authority expects to receive more than $2 million annually in rent and parking fees associated with the project.

They also predicted the hotels­ would require about $140 million in private investment to design and build and generate $5 million a year in tax revenue when they are completed.

The Massachusetts Convention Center Authority’s executive director, James Rooney, said hotel rooms are crucially needed near the South Boston facility to attract the largest trade shows. Currently, he said, some shows avoid Boston because of the cost and inconvenience of busing attendees to the waterfront from hotels in other parts of the city.